Disk drives are information storage devices that use magnetic media to store data and a movable read/write transducer positioned over the magnetic media to selectively read data from and write data to the magnetic media.
To pursue more accurate reading and writing performance, disk drive manufacturers have continued to develop higher capacity disk drives by, for example, increasing the recording and reproducing density of the information tracks on the disks by using a narrower track width and/or a narrower track pitch. As track density increases, it becomes more and more difficult to quickly and accurately position the read/write transducer over the desired information tracks on the disk. One approach that has been effectively used by disk drive manufacturers to improve the positional control of read/write heads for higher density disks is to employ a voice coil motor (VCM). Referring to FIG. 1, a conventional disk drive device using VCM typically has a drive arm 104, a head gimbla assembly (HGA) 106 attached to and mounted on the drive arm 104, a stack of magnetic disks 101 suspending the HGA 106, and a spindle motor 102 for spinning the disks 101. The employed VCM is denoted by reference number 105 and is connected to the drive arm 104 for controlling the motion of the drive arm 104 and, in turn, controlling a slider 103 to position with reference to data tracks across the surface of the magnetic disk 101, thereby enabling the read/write head imbedded in the slider 103 to read data from or write data to the disk 101. Presently, tunnel magneto resistive (TMR) sensor, commonly referred to as TMR sensor, is the prevailing read sensor because of its better capability to read data from media (surface of a disk) at greater track and linear densities than other magneto resistive heads.
There are kinds of characteristics test to measure the quality of a disk drive. SNR is a key parameter for disk drive testing, as it can have good correction with bite error rate, BER. However, total SNR of a disk drive usually includes both “writing” and “reading” effect. In order to have different “writing” and “reading” effect, for the better characterization, two more SNR were proposed:(Total—SNR)2=(Media—SNR)2+(Head—SNR)2 
Concretely, for a storage device comprising a storage media and a head for writing to and/or reading data from the media, the total SNR includes “writing” and “reading” effect, both signal and noise measured under the condition of the head loading on the media with a dynamic fly height (DFH). The head SNR mainly includes read part only, with signal is measured under loading the head and noise is measured by unloading case. And media SNR includes writing effect and media transition related which calculated through below equation:Media—SNR=√{square root over ((Total—SNR)2−(head—SNR)2)}{square root over ((Total—SNR)2−(head—SNR)2)}
In magnetic recording disk drives, where the magnetic recording media on the disks is a granular metal alloy, such as a CoPt alloy, the intrinsic media noise increases with increasing linear recording density. Media noise arises from irregularities in the recorded magnetic transitions and results in random shifts of the read back signal peaks. Higher media noise leads to higher bit error rates. Thus to obtain higher areal densities in magnetic recording disk drives, it is necessary to decrease the intrinsic media noise, i.e., increase the SNR of the recording media. Thereby accurate head SNR is needed for estimating media SNR.
A tradition method for measuring head SNR is using DP (dynamic performance) signal (load on media) while noise value measured by unload condition: both are under “DFH power on”, wherein the DFH power is applying for controlling the DFH. However. However it will under estimate head SNR value because the measuring condition of the head unloading with DFH power on, thereby the environmental temperature during noise measurement will be much higher than the actual case, noise value will become much higher as well.
Hence, it is desired to provide a close loop method to improve head SNR measurement and media SNR measurement accuracy.